Field of the Invention
This invention relates to a fundus examination apparatus such as a laser-Doppler-type, blood-flow meter for measuring the velocity of blood flow in a vessel on the fundus. Further, this invention covers a flow meter for measuring the flow velocities of various fluids.
There is a laser-Doppler, fundus-blood-flow meter as the application of a flow meter. This device irradiates a vessel to be measured of the fundus of an examined eye with a laser beam of a wavelength .lambda., receives the scattered light thereof with a photodetector, detects the interference signal of a Doppler-shifted component of scattered, reflected light from blood cells and scattered light from a stationary vesselcular wall, frequency-analyzes it, and finds the velocity of the blood flow.
Assuming that the scattering intensity of the blood flow in a vessel as the object is proportional to the number of blood cells and that the distribution of the cell density is uniform and the flow thereof is the Poiseuille flow, it is derived that the spectral density of the Doppler shift obtained becomes a flat spectral distribution up to a cut-off frequency .DELTA.fmax corresponding to the maximum flow velocity at the center of the vessel. In the laser Doppler type fundus blood flow meter, this .DELTA.fmax is detected as a physical amount proportional to the maximum flow velocity.
In the bi-directional observation method this cut-off frequency .DELTA.fmax is found relative to signals received from two different directions, whereby the relation between .DELTA.fmax and Vmax is expressed by the angle .DELTA..alpha. formed between two observation directions determined by the construction of the apparatus and the length of the eye axis and the magnitude of the wave number vector k, i.e., 2.pi./.lambda. as shown by the following expression: EQU Vmax={.lambda./ (n-.alpha.)}.multidot..vertline..DELTA.fmax1-.DELTA.fmax2.vertline./cos .delta., (1)
where .DELTA.fmax1 and .DELTA.fmax2 are the maximum shifts of the frequencies calculated from the received signals received by two light receivers, .lambda. is the wavelength of the laser beam, n is the refractive index of the measured region, .alpha. is the angle formed between two light receiving optical axes in the eye, and .beta. is the angle formed between a plane defined by the two light receiving optical axes in the eye and the blood flow velocity vector. By effecting measurement from two directions as described above, the contribution of the incidence direction of the measuring beam is offset, and the blood flow in any region on the fundus of the eye can be measured. Also, by making the angle .beta. formed between the line of intersection between the plane defined by the two light receiving optical axes and the fundus of the eye and the blood-flow-velocity vector coincident, .beta.=0.degree. and the true maximum blood flow velocity can be measured.
When the vesselcular shape and blood-flow velocity of a particular region of the vessel in the fundus of the eye are to be measured, it is necessary for the measuring beam to be accurately positioned onto the target vessel within the measuring time. But actually there are micro eye movements. Therefore, it is difficult to continue to accurately keep the measuring beam onto the target vessel. In order to solve this problem in, the auto-tracking technique of detecting the vesselcular position and moving the position of the measuring beam onto the target vessel in real time corresponding to the micro eye movements is disclosed in Japanese Patent Application Laid-Open No. 6-503733 and Japanese Patent Application Laid-Open No. 7-155299. In these documents, there is adopted a method of irradiating the fundus with a tracking beam from an illuminating light source in a tracking beam optical system and the measuring beam through the intermediary of a rotating mirror at a rotating position, and the design of the device is made such that the spot of the measuring beam irradiates the conjugate point on the fundus of the eye of a tracking reference position on a tracking sensor. The vessel is illuminated by the tracking beam and the image thereof is enlarged and projected onto the tracking sensor, and the rotating mirror is moved so that this vesselcular image may come to the tracking reference position, whereby the measuring beam always continues to irradiate a predetermined vessel.
However, there will be no problem if the tracking-reference-position conjugate point and the measuring beam spot are coincident with each other. But if the tracking-reference-position conjugate point and the measuring beam spot do not coincide with each other due to the structure of the apparatus and the principle of measurement, for example, for the reason that the measuring beam irradiates the fundus at a plurality of different angles, the incidence positions of the tracking beam and the measuring beam differ from each other at the corneal position of the examined eye, and the center of the tracking beam on the vessel and the measuring beam are not coincident with each other.
Further, when there is present great corneal astigmatism or the like, deviation occurs between the center of the tracking beam on the vessel and the measuring beam, and accurate measurement becomes impossible in spite of the tracking system that is operating. In order to solve this problem, an apparatus having correcting means capable of suitably resetting the deviation distance between the center of the tracking beam on the vessel and the measuring beam is disclosed in Japanese Patent Application Laid-Open No. 10-075931. In this example, however, an operator must correct the deviation distance between the center of the tracking beam on the vessel and the measuring beam by visual confirmation and a manual technique and therefore, there is the point to be improved that the correction is greatly affected by not only the operator's skill but the instability of a fixed eye due to conditions such as the operator being sick or advanced in years.
Now, it has heretofore been the ordinary practice to determine each cut-off frequency .DELTA.fmax by the visual judgeluent of the operator. As an improvement over this, there is the technique described in APPLIED OPTICS, vol. 27, No. 6, pp. 1126 to 1134 (1988), "Retinal laser Doppler velocimetry toward its computer-assisted clinical use" (B. L. Petrig, C. E. Riva). Although there is no specific description in this, it is inferred that the cut-off frequency .DELTA.fmax is found by considering an ideal model in which the power spectrum of an FFT waveform vertically falls at the cut-off frequency .DELTA.fmax. The accuracy of this determination of the cut-off frequency increases as the result of frequency analysis approximates the ideal model. That is, the degree of reliability of the measured value can be represented by evaluating the difference from the ideal model. In the above document, after the determination of the cut-off frequency, this difference from the ideal model is evaluated by the use of that cut-off frequency .DELTA.fmax and is regarded as the degree of reliability of measurement.
In the above-described evaluation method, however, it is difficult to obtain a result in a moment because the amount of calculation for determining the final cut-off frequency is vast. Accordingly, when for example, the confirmation of a set measuring condition such as the position of the measuring beam relative to the vessel is to be determined in advance by the quality of a tentatively acquired measurement signal, much calculation time is taken and therefore it is difficult to judge the quality of the condition setting in real time. Further, when use is made of an evaluation method of judging the quality of ti he signal from the shape of its spectrum, if the intensity of the signal component of a Doppler signal is not sufficiently large, the correct measured value from that signal is not provided, shape comparison is not correctly conducted, and the signal is judged to be very good.